How do I mimic the function of the 3D camera in After Effects in Blender?
Welcome to Blender Stack Exchange!

As a quick disclaimer, I'm not a 2D animator nor do I have any real experience with 2D animation, but hopefully I can provide a direction at least.

If I'm understanding what you want to do correctly, you're looking to have animated image sequences on planes, and to add effects like Depth of Field. This is totally possible, and actually pretty easy.

Blender has an addon called Import Images as Planes. It's not enabled by default, so go to Edit>Preferences>Addons>Import-Export: Import Images as Planes, if you're using it for the first time.

Once that is enabled, you can add image sequences to planes by going to File>Import>Images as Planes. From there you can import static images, movie clips, and image sequences, and the planes will be properly scaled, with the image/clip applied as a texture. When importing a sequence of images, make sure you enable the Animate Image Sequences option so that it makes one plane with an image sequence, not as many planes as there are images in the sequence.

Regarding transparency, if your images have an alpha channel, it will use that by default. Otherwise, you can even remove backgrounds using some tricks with materials.

Blender also has depth of field, and it should work as you want it to. Add the depth of field in the camera settings and that should be it. You can even lock the depth of field to a certain object (image) and have it stay in focus and everything else blur.

If I misunderstood what your asking I can edit this answer and expend on it, so don't hesitate to ask!